1. Field of Invention
This invention relates to protective relay systems for electric power transmission circuits and more particularly to directional comparison blocking systems.
2. Background Information
Electric power transmission lines are protected against faults by protective relay systems which operate circuit breakers to isolate the faulted segment of the line. Such systems include protective relays at each terminal of the protected line segment which analyze line currents and voltages to determine the location of a fault and trip the circuit breakers at the respective terminals to isolate a fault determined to be between the terminals.
In one type of protective relay system known as pilot relaying, the protective relays at each end of the transmission line communicate with each other via a bidirectional communication link. The relays at each terminal compare fault location information to determine if the fault is on the intervening line segment. If the comparison indicates that the detected fault is internal, i.e., between the two protective relays, the intervening transmission line section is isolated by tripping the circuit breakers at the line terminals. If the comparison indicates the fault is not between the two protective relays, the circuit breakers remain closed.
The communications channel linking the two relays can employ a modulated carrier signal transmitted over a power line phase conductor or over a microwave system, or audio tones propagated on a leased telephone line or a dedicated pilot wire.
While there are many different types of pilot relaying systems, the blocking pilot relay system is commonly employed. In the traditional blocking system, the carrier signal transmitted over the communications link is used only to prevent one or more of the protective relays from tripping on an external fault. A carrier signal is not required for internal faults, i.e., tripping occurs in the absence of a carrier signal. When the carrier signal is present, the protective relays are blocked from tripping.
To detect a fault with a pilot relaying system, either directional comparison relays or phase comparison relays are used at each terminal. In the directional comparison scheme, fault detecting relays compare the direction of current flow at the two line terminals. Current flow into the line at both terminals indicates an internal fault, and the circuit breakers at each terminal of the line are tripped. If the current flows into the line at one terminal and out at the other, the fault is external and the line remains in service. In addition to the usual forward looking relays in each terminal, the directional comparison blocking scheme is generally composed of, at each terminal, forward looking relays for pilot tripping, reverse looking relays for carrier start, and a carrier auxiliary relay. The performance of the carrier start relay is important, since it affects the operating speed, dependability and the security of the scheme. Traditionally, a separate unit is used.
In general the carrier starting units on a blocking relaying scheme require coordination, on both distance and time, with remote pilot tripping units. In the traditional approach, the carrier start function is performed by a dedicated reverse looking phase distance unit and a non-directional overcurrent ground unit. The characteristic of the distance start unit should include the detection of close in reverse direction faults. Since the forward tripping distance unit does not require this characteristic, this means that a normal distance unit is not suitable for the carrier starting function. Furthermore, the overcurrent ground unit used in the conventional directional comparison blocking scheme may have a problem in some applications, for example, in a severely unbalanced load condition or on an untransposited line system, i.e., a system in which the lines are not twisted and hence a large zero sequence current can flow. Under such conditions, another reverse looking distance unit is needed for carrier starting to replace the overcurrent unit.
U.S. Pat. Nos. 4,484,245 and 4,674,002 disclose a variation on the directional comparison blocking scheme which eliminates the reverse looking unit by using the echo key approach. In the echo key system, the pilot relay initiates the carrier signal for a short duration upon detection of a fault in the forward direction; that is, in the direction of the other terminal. If the pilot relay at the other terminal does not detect a fault in the forward direction, it sends out a carrier signal for a longer duration upon termination of the short duration carrier signal received from the first terminal. Tripping at the first terminal is blocked by the carrier signal of long duration, while the other terminal does not trip since it did not see a fault. The echo key concept is limited to applications in which the echo key circuit is provided at both terminals.
While conventionally, the various relays have been implemented at each terminal by discrete electro-mechanical or electronic relays, microprocessor based relays are now in service in which the various relaying functions are performed by software. In addition to performing relay functions previously performed by discrete relays, the flexibility of the microprocessor based relaying systems has provided the opportunity for additional relaying schemes. For example, the processing capability of digital protective relays has been employed to detect incremental changes in voltage and current which provide much more rapid indication of the onset of fault conditions.
While the present protective relaying systems provide excellent protection for electric power transmission lines, there is always a need for faster more dependable systems.
In particular, there is a need for such improvement in blocking protective relay systems, and in particular, direction comparison blocking relay schemes.
There is a need for such improved directional comparison blocking schemes which are sensitive but which do not overblock.